Recently, a low fuel consumption and a safety of an automobile are strongly demanded in connection with a worldwide effluent control of carbon dioxide associated with a growing interest in environmental problems. In order to cope with such a demand, it is required to reduce a rolling resistance and improve grip performances such as a wet performance, a dry performance and the like as to tire performances. As a means for reducing the rolling resistance of the tire, it is effective to use a rubber composition having a lower loss tangent (tan δ) and a low heat build-up as a rubber composition applied to a tread portion of the tire. It is known that a butadiene rubber traditionally and widely used in the tread portion of the tire or a styrene-butadiene copolymer rubber having a low styrene content and a low vinyl content is exbubbleent in the flexibility at a low temperature since it is low in the glass transition temperature (Tg) as compared with other synthetic rubbers.
However, when the butadiene rubber or the styrene-butadiene copolymer rubber having the low styrene content and the low vinyl content is used in the tread, since the loss tangent (tan δ) of such a rubber composition is low, there is a problem that sufficient gripping performances are hardly obtained.
As a means for obtaining the rubber composition having a low heat build-up, there are considered the decrease in an amount of a filler such as carbon black, silica or the like, the use of carbon black having a large particle size, and so on. In any cases, however, there cannot be avoided the deterioration of the reinforcing property, wear resistance and grip performance on a wet road surface of the rubber composition. As another means for obtaining the rubber composition having a low heat build-up, there are developed many techniques of improving the dispersibility of the filler in the rubber composition. Among them is most effective a method wherein a polymerization active site of a conjugated diene-based polymer obtained through an anionic polymerization with an alkyl lithium is modified by a functional group interactive with a filler (for example, see JP-B-H05-87530, JP-A-H06-49279 and JP-A-S62-207342). However, when the modified conjugated diene-based polymer is used as a rubber component, if a large amount of a softening agent, particularly aromatic oil is compounded, the effect of improving the dispersibility of the filler is not sufficiently developed and there is a problem that the workability, low heat build-up, fracture characteristics and wear resistance of the rubber composition cannot be sufficiently improved. There is also known a technique of providing an excellent flexibility at a low temperature while maintaining a processability, but the effect is not sufficient and at the same time the rolling resistance cannot be improved (for example, see JP-A-H10-53671).
On the other hand, as the rubber composition applied to the tread portion of the tire is preferable a rubber composition having a high storage elastic modulus (G′) for ensuring the grip performance, so that it is demanded to develop a rubber composition having a low loss tangent (tan δ) and a high storage elastic modulus (G′). In this connection, as a means for improving the storage elastic modulus (G′) of the rubber composition is known a method wherein an amount of carbon black compounded in the rubber composition is increased. However, when the amount of carbon black compounded in the rubber composition is increased, the storage elastic modulus (G′) of the rubber composition can be improved, but the loss tangent (tan δ) of the rubber composition is simultaneously increased to deteriorate the low heat build-up of the rubber composition, and further the Mooney viscosity of the rubber composition is raised to cause a problem of deteriorating the processability. Although there is a method of adding a softening agent for improving the processability, the wear resistance and rolling resistance are deteriorated as mentioned above. Therefore, it is increasingly demanded to develop a rubber composition balancing the above conflicting performances.
Particularly, a rubber composition applied to a tread portion of a studless tire is generally required to ensure the flexibility at a low temperature (the term “low temperature” used herein means a temperature during the running on ice and snow and is around −20 to 0° C.), so that it is often one of setting an elastic modulus at a low temperature to a low level by using a polymer having a low glass transition temperature such as butadiene rubber (BR), natural rubber (NR) or the like. However, the rubber composition compounded with the polymer having the low glass transition temperature such as butadiene rubber (BR), natural rubber (NR) or the like is low in tan δ as mentioned above, so that it is difficult to impart sufficient grip performances to the tire. In general, there is a tendency that when the elastic modulus at a low temperature is decreased, an elastic modulus at a high temperature (the term “high temperature” used herein is a temperature affecting the handling during the normal running) is also decreased, so that there is a problem that sufficient grip performances can not be maintained at a high-temperature region in the conventional studless tires. In addition, a sufficient friction coefficient is not obtained on a wet road surface during the normal running, and hence in the studless tire using the above rubber composition it is difficult to improve grip performances on the wet road surface (wet performance) during the normal running.
On the other hand, there have been previously developed various techniques of improving a friction force of a tread rubber itself. As such a technique, for example, there is known a method wherein a rubber composition formed by compounding a foaming agent into a rubber component is used as a tread rubber and then bubbles are generated in a matrix composed of the rubber component by foaming in an adequate manner. For example, JP-A-H09-194640 discloses a method of compounding a foaming agent and a liquid polymer having a weight average molecular weight of several tens of thousands into a rubber component as a technique of improving a wet performance and a grip performance on ice and snow (on-ice performance). However, the surface of the tread rubber obtained by this method is covered with a large number of pores, so that a dewatering effect and an edge effect on the ice and snow road surface can be developed to increase the friction force, while there is a problem that a fracture strength of the tire is deteriorated because the surface is covered with a large number of pores.